1. Field of the Invention
The present disclosure generally relates to an apparatus and associated methodology for improving timing resolution in gamma ray detection. More specifically, the present disclosure relates to an apparatus and associated methodology for improving timing resolution in a gamma ray detection system, such as a positron emission tomography system, by generating timing corrections with respect to an identified interaction location.
2. Discussion of the Background
A commercial gamma ray detector includes an array of scintillator crystals coupled to a transparent light guide, which distributes scintillation light over an array of photomultiplier tubes (PMTs) arranged over the transparent light guide. A position of a gamma ray interaction within the array of scintillator crystals is generally encoded by spreading the optical signal corresponding to the gamma ray over several PMTs grouped into a neighborhood. By measuring relative signal intensities from each of the PMTs in the neighborhood and applying statistical methods or performing a centroid calculation, the location of the gamma ray is decoded.
Signals from the PMTs corresponding to a neighborhood are generally summed in the analog domain, and then timing is measured based on the leading edge of the summed signal. Multiple effects can contribute to relative delays between the PMT signals within the neighborhood. For scintillator crystals with substantially polished surfaces, photons exiting at large angles with respect to the crystal array surface travel significantly farther distances within the crystal than photons exiting at a small angles. These path length differences can be on the order of four times the crystal length, and for high index-of-refraction crystals (approaching or exceeding n=2) path length differences can contribute 200 ps-300 ps of delay for a 10 mm-long scintillator crystal. In a 20 mm-long crystal the delay may be 400 ps-600 ps or more.
Once the scintillation photons reach the output surface of the crystal, there can be significant differences in path length for optical photons traveling to different PMTs. For example, a light guide with an index of refraction of 1.5 may cause a path length difference of 28 mm or more, leading to relative delays among the photon of approximately 140 ps within the light guide itself for photons traveling to two or more different PMTs.
Therefore, the above-described effects combine to yield relative delays of 350 ps to 750 ps or more depending on the specific geometry for photons traveling to several different PMTs. These delays cause a measurable degradation in timing resolution for commercial positron emission tomography (PET) systems, which strive towards timing resolutions of 400 ps or better. Moreover, these relative delays are dependent upon the location of a crystal with which the gamma ray interacts, causing each PMT to lead or lag other PMTs in its neighborhood depending on the position of the crystal with which the gamma ray interacts relative to the PMTs.
In most conventional gamma ray detection systems the timing signal is not derived from digital sampling or multi-threshold sampling. Instead, a composite timing signal is generated by analog summation of signals from a number of PMTs. Then a leading-edge or constant-fraction discriminator is applied to the composite signal. The PMTs whose signals are summed are often referred to as a “trigger zone,” which may overlap or which may be kept separate. Irrespective of the amount of overlap among trigger zones, conventional gamma ray detection systems have fixed trigger zones that are hard-wired on the system's circuit boards. Timing resolution in these conventional systems degrades with increasing count rate because the tails of previous signals on part of the PMTs in a trigger zone interfere with timing detection of subsequent gamma ray-crystal interactions. This interference increases as the average time between interactions decreases, i.e., or the count rate increases.
Gamma ray detection systems that employ digital waveform sampling or multi-threshold sampling are known, but they generally consider only a single photosensor when deriving timing information. A discussion of conventional gamma ray detection systems employing sampling to derive timing information may be found in:
(1) J.-D. Leroux, J.-P. Martin, D. Rouleau, C. M. Pepin, J. Cadorette, R. Fontaine and R. Lecomte, “Time Determination of BGO-APD Detectors by Digital Signal Processing for Positron Emission Tomography”, IEEE Nuclear Science Symposium, Conference Record, 2003, Portland, Oreg.;
(2) R. I. Wiener, S. Surti, C. C. M. Kyba, F. M. Newcomer, R. Van Berg, and J. S. Karp, “An Investigation of Waveform Sampling for Improved Signal Processing in TOF PET”, IEEE Medical Imaging Conference, Conference Record, 2008, Dresden, Germany; and
(3) H. Kim, C. M. Kao, Q. Xie, C. T. Chen, L. Zhou, F. Tang, H. Frisch, W. W. Moses, W. S. Choong, “A multi-threshold sampling method for TOF-PET signal processing”, Nucl. Instr. and Meth. A (2009). Therefore, further discussion of the background art is omitted here for brevity.
The contents of the above-identified documents are incorporated herein by reference.